1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus and an image pickup system using a photoelectric conversion apparatus.
2. Description of the Related Art
Recently, as photoelectric conversion apparatuses have been developed, high-definition and inexpensive digital cameras have become widespread. In particular, the performance of metal oxide semiconductor (MOS) type photoelectric conversion apparatuses, in which each pixel includes an active element and peripheral circuits can be mounted on the same chip, has been significantly improved, and MOS type photoelectric conversion apparatuses are partially replacing charge-coupled device (CCD) sensors. In a MOS type photoelectric conversion apparatus, photodiodes (hereinafter called photoelectric conversion elements) that convert light to electric carriers are arranged in, for example, a two-dimensional array. Each of the photoelectric conversion elements outputs electric carriers to a reading circuit. Electric carriers are first transferred from each of the photoelectric conversion elements to a floating diffusion (FD) and retained in the FD. The gate electrode of an amplifier MOS transistor is connected to the FD, and signals based on the electric carriers in the FD are amplified by a source follower operation and output to an output signal line.
Recently, as the number of pixels has been increased and the sizes have been reduced in photoelectric conversion apparatuses, a reduction in the size of a pixel that includes a photoelectric conversion element is increasingly required even in MOS type photoelectric conversion apparatuses. To this end, a method exists, in which each group of photoelectric conversion elements share a reading circuit that includes an amplifier MOS transistor. A method is disclosed in Japanese Patent Laid-Open No. 2000-232216, in which electric carriers are read from a plurality of photoelectric conversion elements to an FD, and each group of photoelectric conversion elements share an FD and a reading circuit. Moreover, another method is also disclosed, in which each group of photoelectric conversion elements share a reading circuit by connecting a plurality of FDs with a wiring layer.
However, in a method in which an FD and a reading circuit are shared, the layout of photoelectric conversion elements may be asymmetrical. Moreover, since a large FD is formed, the capacitance of the FD becomes large. Moreover, even when a plurality of FDs are connected, parasitic capacitance is formed between metal wiring layers used to connect the FDs and contacts, through holes, or the like for connecting the wiring layers and other wiring lines. Thus, the capacitance of the FDs is increased due to this parasitic capacitance.
When the capacitance of an FD becomes large, a gain obtained when electric carriers transferred from photoelectric conversion elements are output to an output signal line becomes small. When the gain becomes small, the sensitivity of a photoelectric conversion apparatus is decreased, and the signal-to-noise (S/N) ratio of signals is decreased.
Thus, the present invention provides a photoelectric conversion apparatus in which, when a plurality of FDs are connected, the sensitivity is increased, and image signals the S/N ratio of which is improved can be obtained.